BRINGING QUANTUM COMPUTERS DOWN TO SIZE

January 5, 2001

SCIENCE & ENGINEERING NEWS

Ian Austen has reported in the NY Times: Quantum computers rely on the tiniest objects and smallest interactions in the universe. Scientists measure the atomic spin of molecules and manipulate atoms to make these experimental computers, which one day may perform exceptionally complex calculations.

But even though quantum theory tries to describe the tiniest particles that make up matter and how they interact with each other and with energy, experimental quantum computers are anything but small.

“You need a room full of equipment,” said Pierre M. Petroff, a professor of electrical and computer engineering and material science at the University of California in Santa Barbara.

But the world’s dimmest light source, a device created by a team that included Dr. Petroff, may change that. In a paper published in the journal Science on Dec. 22, Dr. Petroff and two colleagues, Atac Imamoglu and Evelyn L. Hu, described a tiny, mushroom-shaped semiconductor that would spit out a single photon, or basic particle of light, on command.

The device may someday allow the development of quantum computers that are based on light particles rather than the movement of atoms. And, perhaps more important, such computers may be the first quantum machines that, at least theoretically, can talk to each other.

“It appears to be the first step to a reliable, single photon source,” said Emanuel H. Knill of the Los Alamos National Laboratory, an author of a paper that appears today in the journal Nature that proposed using photons rather than molecules for quantum computers.

The single photon spitter, which is more formally known as a quantum dot single- photon turnstile device, is based on a structure that Dr. Petroff created seven years ago. The quantum dot, as he put it, is an “electron jail.”

The secret to keeping electrons in captivity is mixing semiconductor materials with different electrical properties on the same surface. Semiconductors, like the silicon in computer chips, become conductors when exposed to different levels of energy.

In the case of the photon device, the researchers built a base of gallium arsenide crystals and then splattered it with atoms of indium arsenide. The relatively poor conducting quality of the base material effectively trapped electrons within the tiny indium arsenide crystals. That is, until an outside force arrived.

In Dr. Petroff’s experiments, that force was a rapidly repeating laser beam. The beam loads the quantum dots with energy, causing them to fire off photons.

At least that was the theory. Unfortunately, the researchers had no way of knowing if that was actually happening with their first device.

The problem, they found, was its pillar shape. Not only was the laser creating activity in the quantum dots, it was also prompting activity in the base material. The radiation from the base overwhelmed the equipment that the researchers had hoped would detect single photons coming from the dots.

It was Dr. Hu who solved the problem by adapting microdisk technology, which was initially developed to build specialized lasers.

For the second version, the quantum dots, also made of indium arsenide, were created within a microdisk of gallium arsenide just 200 nanometers thick. (A human hair can be about 10,000 nanometers thick.) The microdisk then rested on a stem, with the device resembling a mushroom.

On the second model, the researchers focused the laser beam so that it only hit the edge of the microdisk. This time, the sensors, modified versions of devices normally used in fiber optic networks, showed that the device was producing single photons.

“I don’t like to paraphrase God,” Dr. Petroff said. “But it was as if he said, `Let there be a photon’ and there was a photon.”

But problems remain. Because the microdisk is circular, the photons fly off in any direction, making their detection unnecessarily difficult. The researchers hope that a new model, with an elliptical disk, will offer more directed output.

The current device also requires extreme cooling to operate, making the unit the size of a tabletop, despite the tiny proportions of the actual photon emitter. Dr. Petroff said that over time, a much more compact model that works at room temperature would be possible. Quantum computers are based on a different code than the binary system of zeroes and ones that are the basis of conventional computers.

In the atomic and subatomic world of quantum theory, normal rules do not apply. Among other things, that allows quantum computers to be based on units that can represent several states at the same time. Instead of ones and zeroes, experimental quantum computers use qubits, which can represent one, zero or, in theory, any point in between.

Single photons could be transformed into qubits by passing them through polarizing filters like those used by photographers and sunglass makers to control glare. With the polarizers, some photons could be made to vibrate up and down as they travel, others side to side and still others diagonally. Those movements, in turn, could be used to represent different cubits.

Potentially, photons are not likely to be the fastest quantum computer technology, said Dr. Knill at Los Alamos.

But both Dr. Knill and Raymond Laflamme, also a quantum computing researcher at Los Alamos and a co-author of the paper, said that limitation would be more than offset by the ease with which photon-based computers could communicate. Unlike quantum computers that calculate by measuring atomic spin, photon- based machines could exchange data using variations of fiber optics systems that now power the Internet.

Practical photon-based quantum computers are a long way off, Dr. Laflamme said, and all of their potential uses are still unknown.

Right now it appears that they will be better at factoring huge numbers and simulating quantum theory effects than current computers, he said. They also have the potential to offer higher levels of security. But for other uses, he said, quantum computing may not be as good as old-style computing involving electrons.

“This isn’t going to speed up your graphics,” he said.

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